The document discusses predictive parsing and LL(1) grammars. It begins by providing an example grammar and walking through calculating FIRST and FOLLOW sets. It then explains predictive parsing, including that it works for LL(1) grammars where the next input symbol uniquely determines the production rule. The document outlines table-driven predictive parsing, including constructing the parsing table. It provides algorithms and examples for predictive parsing and reconstructing the parse tree. Finally, it discusses properties of LL(1) grammars and different error recovery techniques for predictive parsing.

1. SYNTAX
ANALYSIS
OR
PARSING
Lecture 06

2. FOLLOW EXAMPLE
 S  a S e | B
 B  b B C f | C
 C  c C g | d | ε
 FIRST(C) =
 FIRST(B) =
 FIRST(S) =
 FOLLOW(C) =
 FOLLOW(B) =
 FOLLOW(S) = {$}
Assume the first non-terminal is the start symbol
1. If A is start symbol, put $ in FOLLOW(A)
2. Productions of the form B  α A β,
Add FIRST(β) – {ε} to FOLLOW(A)
3. Productions of the form B  α A or
B  α A β where β ⇒*
ε
Add FOLLOW(B) to FOLLOW(A)
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3. FOLLOW EXAMPLE
 S  a S e | B
 B  b B C f | C
 C  c C g | d | ε
 FIRST(C) = {c,d,ε}
 FIRST(B) = {b,c,d,ε}
 FIRST(S) = {a,b,c,d,ε}
 FOLLOW(C) =
 FOLLOW(B) =
 FOLLOW(S) = { }$, e
{c,d} ∪ FOLLOW(S)
= {c,d,e,$}
{f,g} ∪ FOLLOW(B)
= {c,d,e,f,g,$}
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4. PREDICTIVE PARSING
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5. PREDICTIVE PARSING
 LL(1) Grammars
 Can do predictive parsing
 Can select the right rule
 Looking at only the next 1 input symbol
 First L : Left to Right Scanning
 Second L: Leftmost derivation
 1 : one input symbol look-ahead for predictive decision
 LL(k) Grammars
 Can do predictive parsing
 Can select the right rule
 Looking at only the next k input symbols
 Techniques to modify the grammar:
 Left Factoring
 Removal of Left Recursion
 LL(k) Language
 Can be described with an LL(k) grammar
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6. TABLE DRIVEN PREDICTIVE
PARSING
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7. PARSE TABLE CONSTRUCTION
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8. TABLE DRIVEN PREDICTIVE
PARSING
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9. TABLE DRIVEN PREDICTIVE
PARSING
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10. PREDICTIVE PARSING
ALGORITHM
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11. PREDICTIVE PARSING
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12. PREDICTIVE PARSING
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13. PREDICTIVE PARSING
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43. PREDICTIVE PARSING
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55. PREDICTIVE PARSING
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56. PREDICTIVE PARSING
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57. PREDICTIVE PARSING
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58. RECONSTRUCTING THE PARSE
TREE
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59. RECONSTRUCTING THE PARSE
TREE
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60. RECONSTRUCTING THE PARSE
TREE
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61. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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62. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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63. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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64. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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65. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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66. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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67. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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68. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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69. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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70. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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71. EXAMPLE: THE “DANGLING ELSE”
GRAMMAR
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72. LL(1) GRAMMAR
 LL(1) grammars
 Are never ambiguous.
 Will never have left recursion.
 Furthermore...
 If we are looking for an “A” and the next symbol is
“b”,
 Then only one production must be possible
 Although elimination of left recursion and left
factoring is easy.
 Some grammar will never be a LL(1) grammar.
LL(1) Grammar
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73. LL(1) GRAMMAR
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74. PROPERTIES OF LL(1) GRAMMAR
 A grammar G is LL(1) if an only if whenever A
→α | β are two distinct productions of G the
following conditions hold:
1. For no terminal a do both α and β derive strings
beginning with a.
2. At most one of α and β can derive the empty string.
3. If then β ⇒* ε , then α does not derive any string
beginning with a terminal in FOLLOW(A).
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75. ERROR RECOVERY
a + b $
Y
X
$
Z
Input
Predictive Parsing
Program
Stack Output
Parsing Table
M[A,a]
When Do Errors Occur? Recall Predictive Parser Function:
1. If X is a terminal and it doesn’t match input.
2. If M[X, Input] is empty – No allowable actions 75

76. ERROR RECOVERY
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77. ERROR RECOVERY: SKIP INPUT
SYMBOLS
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78. ERROR RECOVERY: POP THE
STACK
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79. ERROR RECOVERY: PANIC MODE
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80. ERROR RECOVERY - TABLE
ENTRIES
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81. QUESTIONS ?
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